This invention relates to the development of copper-silver-titanium brazing filler metals (or braze alloys) for direct brazing of ceramics to ceramics or ceramics to metals to form strong joints that can withstand high service temperatures. It was developed under a contract with the United States Department of Energy.
A key technology that will enhance or restrict the use of ceramic materials in high performance applications, such as advanced heat engines or high temperature heat exchangers, is the ability to reliably join simpleshape ceramic components to form complex assemblies or to join unit lengths of ceramic material to form large ceramic systems. Although ceramic joining technology has been highly developed over the past fifty years, very little has been done to develop brazing filler metals for joining ceramics for use at elevated temperatures, at high stress levels and in dirty environments.
There are basically two brazing processes that can be used for joining ceramics. One is indirect brazing in which the ceramic is coated with a metal such as molybdenum or titanium prior to brazing with a nonreactive commercial filler metal. In the widely used moly-manganese process a glassy phase forms a bond with the ceramic and the molybdenum particles that constitute the coating. In other indirect processes, the active metal titanium is applied to the ceramic by vapor deposition or from a slurry containing titanium hydride. The other major ceramic brazing process is direct brazing with filler metals specially formulated to wet and bond to the ceramics. Direct brazing is preferred since it avoids the development and application of what is, in many cases, the very sophisticated and expensive coating or metallizing treatment required for indirect brazing. Also, the inclusion of an active metal such as titanium within the filler metal more effectively protects the active metal from oxidation during storage or while brazing than when the pure active metal is first used to coat the ceramic. Finally, the strength of the bond between a coating and ceramic substrate, and the corrosion resistance of the coating do not have to be of concern in direct brazing.
In spite of the potential advantages of the direct brazing process, there are very few filler metals commercially available that will wet and adhere to structural ceramics. It is difficult to formulate such an alloy since ceramics, and particularly oxide-base ceramics, are notoriously difficult to wet. One filler metal that has been developed to braze ceramics is made by the WESGO Division of GTE Products Corporation and sold under the trade name "Ticusil". This alloy contains 55 at. % silver, 36 at. % copper and 8 at. % titanium. However, the strengths of ceramic-ceramic brazements made with this filler metal tend to be low, and quite variable even within a given brazed sample. Therefore, in order to meet the requirements for joining various ceramics for high performance applications, there is a need to develop filler metals that can wet and strongly adhere to ceramics and also withstand high service temperatures and oxidizing conditions.